P-keratin

In the early 1930s W. T. Astbury and his coworkers observed that the stretched, moist hair showed a drastic change in its X-ray diffraction pattern, compared with the dry, unstretched hair. This was interpreted as two forms of the pol q)eptide chain. One was the extended form, p-keratin, eventually called the P-pleated sheet. The other was the coiled form, a-keratin, eventually called the a-helix. 

The more hydrophobic homolog, decylmethylsulfoxide (DCMS), has been shown to produce a greater level of a-to-P-keratin conversion, compared to DMSO, and as deduced by FT-IR [109]. Correspondingly, DSC studies have revealed that DCMS has a similar effect to DMSO on the stratum comeum thermogram, but at greatly reduced concentrations [79]. 

When the fibre is stretched, the coil extends to give p- keratin and on releasing the keratin back into a-form [48]. The amino acid and sequence in wool varies with the variety of wool [49]. The average amino acid contents for major varieties of wool and silk are given in Table 1.8. R represents relatively large side chains which contain amino, carboxyl, sulphide groups and relatively 50% of the weight of wool... 

Miettinen M, Kovatich AJ, Karkkainen P. Keratin subsets in papillary and follicular thyroid lesions A paraffin section analysis with diagnostic implications. Virchows Arch. 1997 431 407-413. 

Structural proteins comprise the matrix for bone and hgaments, and include collagen and elastin. They also provide strength and elastidty to the internal organs, and to the vascular system. The protein p-keratin plays a role in the structure of epidermal tissue. 

In humans, perhaps the best-known example of an aggregated P-sheet containing fibrous protein is P-keratin, an aggregated P-sheet-containing protein. P-Keratin is the major constituent of human nail, bird feather calamus and rachis, and reptile scales and claws. This differs from amyloids by having a P-structure in which the peptide chains are parallel, rather than perpendicular, to the direction of the fibril axis. 

Insects and arachnoids produce well-known amyloids. Silk and spider webs, like P-keratin, also differ from amyloids in being fibrous P-sheet proteins composed of peptide strands that are parallel, rather than perpendicular, to the direction of the fibril axis. For the process of silk formation by spiders, it has been proposed that fibrils in the silk gland have an initial cross-P structure (Kenney et al. 2002 Table 3) that, when stretched, assume parallel P-structures. However, X-ray diffraction for a peptide derived from the central domain of the A class of chorion proteins, derived frovaAntheraea polyphemus eggshells, displayed P-sheets perpendicular to the fibril axis, the same cross-P structure that occurs in amyloid proteins (Iconomidou et al. 2000 Table 3). The stability and strength of the amyloid fibres provides mechanical and biological protection for the oocyte and developing embryo from a variety of environmental and predatory hazards. 

Figure 9.4 Animal hair and spiderwebs are constructed of the fibrous proteins a-keratin and P-keratin, respectively. What properties of fibrous proteins make hair, fur, and spiderwebs useful ...

Fibrous proteins, being insoluble in water often have a structural or protective function. The most familiar fibrous proteins are the keratins and collagen. a-Keratin (Figure 25.18) is based on the a-helix secondary structure and is the protein structural component of hair, wool, nails, claws, quills, horns, and the outer layer of skin. P-Keratin is based on the P-sheet secondary structure and occurs in silk as fibroin. L-Cysteine is especially abundant in keratins, where it can account for more than 20% of the amino acids present. Collagen occurs mainly in connective tissue (cartilage and tendons) and has a triple helix structure. 

Nail is also keratinized tissue, but it is harder than SC. This is partly due to the hard a-keratin in nail as opposed to the more soft p-keratin in SC (Baden, 1970 Forslind, 1970). 

The X-ray diffraction photographs of feather rachis keratin, which have previously been interpreted as involving P-keratin pleated sheets and a-keratin helixes in molecular distribution, may show a superimposed P-keratin pattern and a-keratin pattern. If this is correct, the a-keratin is different in nature from hair and horn a-keratin, probably consisting of AB cables with ropes (three a-helixes coiled about one another) in the interstices. This structure accounts in a striking way for the characteristic features of the X-ray photographs. 

Similar infrared research on P-keratin and collagen showed that most of the NHO hydrogen bonds in their structures must be oriented laterally, at least approximate y perpendicular to the chain axes. 

I have dealt in some detail with P-keratin models in which the dihedral angle between the two planes of hydrogen bonds passing through each chain axis is about 120 degrees. Some of these models are assemblages of "6-stacks" others are composed of sheets. Further consideration should be given to 1-residue-per-turn models for P-keratin that are composed of 3-stacks, as I had previously proposed. See Fig. 1,... 

Keratins are classified into two types. The a-keratins are the main proteins of wool and hair. The P-keratins occur in feathers, skin, beaks and scales of most birds and reptiles. These proteins are very rich in the sulphur-containing amino acid cysteine wool protein, for example, contains about 4 per cent of sulphur (see p. 373). 

The a- and p-keratins exist in the oriented crystalline state and possess a high concentration of the cystine residues. They also undergo contraction when subjected to the action of a wide variety of reagents.(70) It is recognized that two distinctly different types of contractile processes can be observed in a-keratin fibers. One of these involves the interaction with reagents known to sever disulfide cross-links. As would be expected, in this case the observed dimensional changes are irreversible. 

The high glycine content allows tight packing of the fibers to increase strength and resistance to stretching. Keratin refers to a family of fibrous structural proteins. When assembled in bundles, it is tough and insoluble, and is found in reptiles, amphibians and mammals. The a-keratins are the scientific name for hair, such as wool, horns, and human hair. It is called alpha because of its majority structure in the conformation of an a-helix. The harder p-keratins found in nails, claws and shells (turtle) form P-sheets. 

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